JP2005-295248 A discloses a high-frequency oscillatory proximity sensor, which makes a contactless detection of a metal-made object (an electrical conductor) or an object of magnetic material. The proximity sensor includes a parallel L-C resonator composed of a capacitor and a detection coil which is caused to oscillate in the absence of the object within a detection range. Also included in the proximity sensor are an oscillator driving the resonator to oscillate and an oscillation detector which detects an amplitude of the oscillation and provides a corresponding output voltage for determination of whether or not the object is present based upon the level of the output voltage.
In view of that the detection coil may be broken or burnout for some reasons, it has been demanded to make an early detect ion of the burnout of the detection coil. Such scheme of detecting the burnout has been proposed, for example, in JP2000-132226 A.
The scheme requires the steps of converting the sensor output into a corresponding voltage, and comparing the voltage with a threshold for determination of the burnout.
A straightforward modification could be possible to apply the above scheme into the proximity sensor of JP2005-295248 such that the burnout detection can be determined by a comparison of the voltage of an output voltage from the oscillation detector with a predetermined threshold.
However, the above modification would suffer from a drawback in that the oscillation detector is difficult to discriminate the burnout of the coil from a detection of the object in a certain condition where the L-C resonator sees a slight oscillation, thereby failing to detect the burnout reliably and independently from the detection of the object. This occurs in the above mentioned proximity sensor in which a bias current flows from the oscillator into the ground such that the oscillation detector acknowledges a voltage caused by the flow of the bias current as if it comes from the oscillation of the L-C resonator, even when the detection coil is broken or burned out to stop the oscillation. The voltage thus generated as a consequence of the burnout of the detection coil cannot be distinguished from the voltage caused by the oscillation of the L-C resonator.
While the L-C resonator keeps oscillating with the unbroken detection coil, the voltage across the resonant capacitor constantly shows a sinusoidal waveform of several volts, or an AC voltage having an amplitude in proportion to the distance between the object and the detection coil while the L-C resonator keeps oscillating. This means that the oscillation detection signal obtained by rectification of the AC voltage is always generated while the L-C resonator keeps oscillating. In case the detection coil is broken, the bias current from the oscillator does not pass through the detection coil, but is caused to flow towards the ground by way of transistors and resistors included in the oscillator. Accordingly, even when the detection coil is broken, the output signal shows a certain voltage which is almost the same as the voltage generated when the L-C resonator keeps oscillating in a certain condition.
In other words, the proximity sensor sees only a slight voltage difference between when the oscillation becomes small and when the detection coil is broken. In view of that the proximity sensor functions inherently to vary the oscillation voltage in proportion to the distance between the object and the detection coil, it is difficult to definitely distinct two thresholds (i.e., individual reference, voltages) relied upon respectively for detection of the nearby object and determination of the burnout of the detection coil, thereby failing to realize practically available detection accuracy.